This invention relates to heaters for liquids, and particularly aquarium heaters.
Aquarium heaters conventionally comprise wire-wound heating elements such as resistance heating wires wound about a hollow ceramic core. The heating element is in series with a bi-metal thermostat which is set by means of a knob to open when the temperature of the air around the thermostat exceeds a selected temperature in the range of typically 25.degree. to 45.degree. C. The heating element is housed in a cylindrical glass container, such as a test tube.
Although these aquarium heaters are the standard of the industry, they often malfunction in use. In particular, the glass housing is relatively fragile and can develop cracks. Mechanical abuse can cause cracks or fractures in the housing. A cracked housing can also result from operational abuse, such as operating the heater with a low water level in the aquarium, and then filling the aquarium with cold water, thereby thermally shocking the glass. Because water can then leak into the housing, current leakage can occur, which can result in electrocution of fish in the aquarium, and sometimes even a shock to an ichthyologist.
Another problem with conventional aquarium heaters is that the bi-metal thermostat can malfunction, usually in the closed mode or can by human error be set in the fully closed mode. If this occurs, the fish can be killed.
Another form of operational abuse that occurs is operation of the aquarium heater when it is not immersed in water, such as when the heater is removed from the aquarium for cleaning of the aquarium. The heater can then be a source of fire because the heater can get much hotter in air than in water. In addition, if the hot heater is immersed in water, breakage of the glass housing can occur, with excessive leakage current resulting.
In response to these problems with electric heaters for aquariums, Underwriters Laboratories, Inc. recently promulgated UL 1018 standard regarding electric aquarium equipment. This standard, which took effect on Jan. 1, 1979, requires for UL approval that in normal operation, aquarium heaters have a leakage of current which does not exceed 0.5 milliamperes. Under abnormal use, a heater is required to: (1) be able to be dropped three times from a height of three feet off a hardwood surface; (2) be suspended from a three foot length of flexible cord and released so that it swings by the cord against a vertical wall of hardwood; and (3) be able to take a crushing force using a 12-inch square flat steel plate with a crushing load of 100 lbs. force. In spite of such abuse, the heater is required to remain intact with a leakage current of no more than 0.5 milliamperes. Aquarium heaters with a glass housing cannot pass these mechanical abuse tests.
There are other UL tests that heaters with glass enclosures cannot satisfy. For example, an immersible heater with its control set for maximum heating is required to be able to operate in free air until it is well heated and then dipped suddenly into water at room temperature. The aquarium heater is required to undergo this test five times, and still exhibit no dielectric breakdown, demonstrate no adverse mechanical or operational effects, and have no increased likelihood of fire, electric shock or injury to a person.
Another UL test that conventional heaters cannot pass is the fire test. In this test, the heater is placed on a softwood board. The heater is covered with cheesecloth, set to maximum heat, and operated for 7 hours. No glowing or flaming of the cheesecloth or tissue paper is permitted.
It is evident that there is a need for an immersible heater suitable for use in aquariums, where the heater performs satisfactorily, not only in normal operation, but also when subjected to missuse and abuse in ordinary and extraordinary operation.